Chromosome

A chromosome is a long, tightly coiled package of DNA found inside the cells of living things. DNA is the molecule that carries the instructions for building and running a body. Chromosomes are the way cells keep that DNA organized and easy to copy. Almost every living thing on Earth, from oak trees to octopuses to people, stores its DNA inside chromosomes.

Each chromosome is made of one very long DNA strand wrapped around tiny bundles of protein. The DNA gets folded and twisted until it fits inside a space too small to see without a microscope. If you unwound the DNA from a single human chromosome, it would stretch several inches long. Folded up, it is smaller than a speck of dust.

Different living things have different numbers of chromosomes. Humans have 46, arranged in 23 pairs. You got half from your mother and half from your father. Dogs have 78. Fruit flies have only 8. The record holder is a small fern called Ophioglossum, which has more than 1,200. Having more chromosomes does not mean an animal or plant is more complex. It is just a different way of packing the instructions.

Along each chromosome are sections called genes. A gene is a short piece of DNA that tells the cell how to make one thing, like the color of your eyes or the shape of a tomato leaf. Humans have about 20,000 genes spread across their 46 chromosomes. Scientists once thought humans would have many more, since we are complex animals. The discovery that we have roughly the same number as a mouse surprised researchers and is still being studied.

Two of your chromosomes are called sex chromosomes. In most humans, they are either XX or XY, and they help decide many traits related to biological sex. The other 44 chromosomes are the same type in everyone.

Chromosomes matter most when cells divide. Before a cell splits in two, it copies every chromosome so each new cell gets a full set. If a copy has a mistake, or if a chromosome breaks or gets lost, problems can follow. Some genetic conditions happen when a person has an extra chromosome or a missing one.

Chromosomes were first seen under microscopes in the 1840s. Their name comes from Greek words meaning "colored body," because they soaked up the dyes scientists used to stain cells.